This invention pertains to retroreflective materials and most particularly retroreflective material using micro cube corner prisms as the retroreflective elements.
Retroreflective materials are employed for various safety and decorative purposes. Particularly, these materials are useful at night time when visibility is important under low light conditions. With perfect retroreflective materials, light rays are reflected towards a light source in a substantially parallel path along an axis of retroreflectively. For many applications, perfect retroreflectivity is not required. Rather, a compromise is required in which a cone of divergent light is retroreflected which permits enough light to strike the viewer's eye, yet not so much that the intensity of the reflective light at the viewer's eye is unduly diminished. Under circumstances where the only source of illumination is the headlights of an automobile on an unlit road, the ability to retroreflect such a cone of divergence to the eye of the driver is important for safety reasons.
Many types of retroreflective material exist for various purposes. These retroreflective materials can be used as reflective tapes and patches for clothing, such as vests and belts. Also, retroreflective bands can be used on posts, barrels, traffic cone collars, highway signs, warning reflectors, etc. Retroreflective material may be comprised of arrays of randomly oriented micron diameter spheres or close packed cube-corner (prismatic) arrays.
Cube-corner or prismatic retroreflectors are described in U.S. Pat. No. 3,712,706, issued to Stamm (Jan. 23, 1973). Generally, the prisms are made by forming a master negative die on a flat surface of a metal plate or other suitable material. To form the cube-corners, three series of parallel equidistance intersecting V-shaped grooves 60 degrees apart are inscribed in the flat plate. The die is then used as a mold to form a transparent cube-corner array which is then processed into sheets of retroreflective material.
When the groove angle is 70 degrees, 31 minutes, 43.6 seconds, the angle formed by the intersection of two cube faces (the dihedral angle) is 90 degrees and the incident light is reflected back to the source. For automobile headlight reflectors, the dihedral angle is changed slightly so that the incidental light is reflected non-orthogonally towards the driver instead of the source.
Preferably, the retroreflected light from the vehicle headlights should be returned in a cone wide enough to encompass the eye of the vehicle's driver (this angle is referred to as the angle of observation).
At long distances the cone of light need only encompass two-tenths of a degree, but as the distance is decreased and/or as the distance from the head lamps to the eyes of the driver increase (as in the case of the driver of a large truck verses that of a sports car) then the cone of light should be increased to five-tenths or even one degree.
Many attempts have been made to keep the intensity of the retroreflected light uniform over this larger cone. Changing the dihedral angle of the cube corner prism will spread this cone of light, but in a star shaped pattern that is not uniform.
Diffraction of the light (see Stamm U.S. Pat. No. 3,712,706) by the small effective aperture of the cube corner prisms spreads the light, but again in a non-uniform manner with hot spots and nulls in decreasing intensity as the angle of the cone increases.
Mild diffusers have been tried such as texturing the front surface of the material or incorporating light scattering pigments or light refracting particles in a top coating on the front surface or in a top film. This technique scatters or redirects the light over much larger angles beyond the viewing cone so that much of the light is lost.